1. Field
The present invention generally relates to techniques for charging a battery. More specifically, the present invention relates to a technique for charging a lithium-ion battery, wherein the technique controls a lithium surface concentration in the battery based on measurements of current, voltage and temperature during the charging process.
2. Related Art
Rechargeable lithium-ion batteries are presently used to provide power in a wide variety of systems, including laptop computers, cordless power tools and electric vehicles. FIG. 11 illustrates a typical lithium-ion battery cell, which includes a porous graphite electrode, a polymer separator impregnated with electrolyte, and a porous cobalt dioxide electrode. The details of the transport of lithium and lithium ions in and out of the electrode granules and through the material between them are complex, but the net effect is dominated by slow diffusion processes for filling one electrode with lithium while removing it from the other.
Note that FIG. 11 provides a physical model for the layout of a typical lithium-ion cell, wherein the oxidation and reduction processes that occur during charging are also illustrated. The physical model shows the current collectors, which are in turn connected to the battery terminals; the polymer separator; and the positive and negative porous electrodes. Note that an electrolyte permeates the porous electrodes and the separator.
The negative electrode includes granules of graphite held together with a conductive binder (in practice, there may also be a nonconductive binder). Surrounding each graphite particle is a thin passivating layer called the solid-electrolyte interphase (SEI) that forms when a fresh cell is charged for the first time from the lithium atoms in the graphite reacting directly with the electrolyte. This occurs because the tendency for the lithium atoms to remain in the graphite is relatively weak when the cell is fully charged, but after the SEI is formed, the SEI acts as a barrier against further reactions with the electrolyte. Nevertheless, the SEI still allows transport of lithium ions, albeit with some degree of extra resistance.
The positive electrode includes granules of lithiated cobalt dioxide held together with binders similar to the negative electrode. Any SEI-like layer surrounding these particles is likely to be of much less significance than in the negative electrode because lithium atoms strongly favor remaining in these particles rather than leaving and reacting directly with the electrolyte.
Lithium transport in the negative graphite electrode (also referred to as the “transport-limiting electrode”) is slower than in the positive cobalt dioxide electrode (also referred to as the “non-transport-limiting electrode”), and therefore limits the maximal speed of charging. During charging, the slow diffusion causes a transient build-up of lithium on the surfaces of the graphite that varies in direct proportion to the charging current and a characteristic diffusion time.
The diffusion time is typically on the order of hours and has a strong dependence on temperature and other variables. For instance, a cell at 15° C. can have a diffusion time which is ten times slower than a cell at 35° C. The diffusion time can also vary significantly between cells, even under the same environmental conditions, due to manufacturing variability.
If the concentration of lithium at the surface reaches the saturation concentration for lithium in graphite, more lithium is prevented from entering the graphite electrode until the concentration decreases. A primary goal of conventional battery-charging techniques is to avoid lithium surface saturation, while keeping the charging time to a minimum. For example, one conventional technique charges at a constant current until a fixed upper voltage limit (e.g., 4.2 V) is reached, and then charges by holding the voltage constant at this upper limit until the current tapers to some lower limit. Note that it is common practice to express all currents in terms of the cell capacity. For example, for a cell with a capacity of Qmax=2500 mA·hr, a “1 C” current would be 2500 mA. In these units, the constant current charging is usually done at less than 1 C (e.g., 0.3 C), and the constant voltage phase is terminated when the current tapers to some value less than 0.05 C.
A significant challenge in charging lithium-ion batteries is to avoid lithium surface saturation at a transport-limiting electrode, while keeping the charging time to a minimum. Some battery-charging techniques, such as Adaptive Surface Concentration Charging (ASCC), avoid lithium surface saturation during the charging process by adapting to the dynamics of lithium transport in a battery through closed-loop control of estimated single electrode potentials (or, equivalently, estimates of lithium concentration at the surfaces of the electrodes.) For example, see U.S. patent application Ser. No. 12/242,700, filed 30 Sep. 2008, entitled “Adaptive Surface Concentration Battery Charging,” by inventors Thomas C. Greening, P. Jeffrey Ungar and William C. Athas, which is hereby incorporated herein by reference.
During the charging process, one implementation of ASCC estimates the electrode potentials from the cell's state of charge, the measured single electrode potential curves for a typical cell, and cell impedance data. Note that these ASCC techniques are more general than any specific model used to estimate the electrode potentials, because they account for the fact that at any state of charge there is a minimum charging voltage, where the cell will not charge, and a maximum charging voltage, where the cell will be overdriven and experience a shortened service life. These are illustrated by the “no charging” and “maximum charging” lines on FIG. 1. The desired target charging voltage is shown as a line between these limits. For example, charging at the cell's open circuit voltage (OCV) will come to a halt once transients have decayed. On the other hand, charging at the maximum limit voltage may drive a graphite negative electrode to conditions where the lithium surface concentration quickly reaches saturation. These two extremes define the maximum margin for error in the fractional state of charge q in the neighborhood of an estimated q, shown as Δq in FIG. 1.
Note that the full state of charge margin Δq is typically greater than 10% over most of the state of charge range, although it can be narrower than this at high states of charge, depending on the chemistry and the loading of the electrodes. Some of this margin can be used to compensate for cell manufacturing variations, including localized variations in internal cell properties. Furthermore, if the estimated state of charge is lower than the true value, the calculated target charging voltages will be closer to the “no charging” line, which results in long and highly variable charging times. For best performance, the state of charge should be known to within approximately ±1%.
Unfortunately, this requirement is beyond the capabilities of commonly available battery systems. These systems employ bookkeeping strategies to combine state of charge measurements (based on relaxed OCV values) with coulomb counting and the coulomb capacity Qmax. However, a host of issues, including cell OCV hysteresis, slow cell relaxation, lack of cell isolation, and infrequent updates to Qmax, prevent them from doing much better than ±4% in practice.
Hence, what is needed is a technique that does not require accurate measurements of a battery's state of charge in order to charge consistently and to keep the electrode lithium surface concentrations (and potentials) within specified limits, at least for typical cells that meet or exceed certain transport rate capabilities.